Summary

In the Fourth Objections, French philosopher Antoine Arnauld claims that clear and distinct knowledge of one's mind does not exclude one's body from one's essence. He makes a comparison to a geometric calculation to clarify his point. One can know that a triangle can be formed from a single right angle without knowing the Pythagorean Theorem that explains the properties of the figure. The reason for the lack of knowledge could be that the individual is in doubt, does not yet understand, or has been misled. Similarly, one can know one's mind without also knowing one's body. It is possible one's knowledge is simply inadequate, such that excluding body from one's essence could be an error.

Descartes agrees there may be features of himself about which he is not aware, but this does not mean these features are essential. Indeed, since he believes he has proven he can exist without the body, this implies it is not essential to his existence.

Analysis

Arnauld's objections are considered important by philosophers, not only for their own insights but also because they help Descartes clarify his ideas. It seems as if Arnauld's point is epistemological, whereas Descartes's is metaphysical. Both Arnauld and Descartes agree there are aspects of oneself one may not yet know—this is the epistemological point. Descartes, however, rejects Arnauld's example of the triangle and the Pythagorean Theorem, saying the example is not analogous to the mind and body. The triangle exists, but the theorem simply explains the triangle. In contrast the mind and body are two separate things, each of which can be understood separately.